'44 



Cincinnati Society of Natural History. 



posterior umbonal ridges, which sinus is directed backward, 

 and gradually widening, it terminates in a broad contraction 

 of the basal margin a little anterior to the middle of the shell. 



Surface of the valves marked by concentric lines of growth, 

 which appear as deep undulations at the anterior end of the 

 shell. There is some evidence of fine radiating lines on the 

 posterior umbonal slopes, but they are too indefinite on our 

 specimens to be shown in the illustrations. Our specimens 

 are casts, none of the shell itself is preserved. There was no 

 gaping of the shell at either end. 



This species in some respects might be compared with 

 Ort/iodesma rectum, the type of the genus, but it is much 

 more closely related to Orthodesma mickelboroughi. It is true, 

 that the long subfusiform shape at once distinguishes it from 

 O. mickelboroughi, but that is nearly all the difference there is 

 between them. Suppose you could take hold of the postero- 

 basal angle of an O. mickelboroughi and draw the shell out, by 

 taking up the basal margin, without lengthening the cardinal 

 line, or seriously disturbing it anterior to the beaks, until the 

 shell is increased one-half in length, you would produce a 

 shell very much like the species here described. There 

 seems to be no substantial difference between the species, 

 except in the relative proportions of the shells. So impressed 

 were we with this fact, that we laid Whitfield's definition of O. 

 mickelboroughi before us, and followed it when writing the 

 description of this species, so that any one can the more 

 readily make the comparison. Had O. mickelboroughi to stand 

 alone on the type specimen, there might be a suspicion that 

 it was founded on an abnormal specimen, but such is not the 

 case, for one of the authors has two specimens, one of which 

 is better than the type, and proves that the species was 

 founded on a normal cast. There was no gaping at either 

 end of O. mickelboroughi. When this species is compared 

 with O. rectum, the shorter hinge line, tapering posterior end 

 and surface undulations will readily distinguish it, but such 

 differences are not of generic importance, nor higher than 

 ordinary specific differences among the casts of the shells of 

 Lamellibranchiata. 



In a recent publication, by the Geological Survey of Minne- 

 sota, a new generic name, spelled Rhytimya, has been pro- 



